Taxol is a naturally occurring diterpenoid which has demonstrated great potential as an anti-cancer drug. Taxol was first isolated and its structure reported by Wani, et al., in "Plant Anti-Tumor Agents. VI. The Isolation and Structure of Taxol. A Novel Anti-Leukemic And Anti-Tumor Agent From Taxus Brevifolia," J. Am. Chem. Soc., 1971, 93, 2325. Taxol is found in the stem bark of the Western Yew, Taxus brevifolia, as well as in T. baccata and T. cuspidata.
The biological activity of taxol is related to its effect on cell division. Taxol promotes formation of microtubules that form the mitotic spindle during cell division. However, taxol prevents depolymerization of the tubulin forming the microtubules of the mitotic spindle, which is essential for cell division to take place. Thus, taxol causes cell division to stop. Taxol's mechanism is unique since it promotes the formation of tubulin polymers, whereas other anti-cancer drugs, such as vinblastine and colchicine, prevent microtubule formation.
Extensive testing of taxol has been a slow process because the drug is in short supply, and it has not yet been successfully synthesized. However, studies have been completed by McGuire et al., that demonstrate that taxol shows excellent clinical activity against drug-refractory ovarian cancer. See "Taxol: A Unique Antineoplastic Agent With Significant Activity In Advanced Ovarian Epithelial Neoplasms," Ann. Int, Med., 111, 273-279 (1989). Another study by Holmes, et al., demonstrates that taxol is an active drug in the treatment of metastatic breast cancer. See "Phase II Trial of Taxol, An Active Drug In The Treatment Of Metastatic Breast Cancer," J. Natl. Cancer Inst., 83, 1797-1805 (1991). All references cited herein are incorporated by reference as if reproduced in full below.
In both of these studies, taxol had to be administered by 24-hour infusions to avoid problems from allergic reactions due to the polyethoxylated castor oil diluent (known by the tradename Cremophor EL) used in the formulation of taxol. The diluent is required because of the low water solubility of taxol. If a prolonged infusion and premedication with antiallergic drugs is not used, severe allergic reactions and even death have resulted. See Weiss et al., "Hypersensitivity Reactions From Taxol, "J. Clin. Oncol., 8, 1263-1268 (1990). For these reasons, the preparation of derivatives of taxol which are more water soluble than taxol, and which retain at least some of the antineoplastic activity of the parent drug is an important objective.
The biological activity of taxols substituted at the C-2' and C-7 positions in order to make them more water soluble has been reported See Magri and Kingston, "Modified Taxols, 4 . Synthesis And Biological Activity Of Taxols Modified In The Side Chain," J. Nat. Prod., 51, 298-306 (1988). A 2'-(t-butyldimethylsilyl) taxol was synthesized and found to be essentially inactive; this was taken as an indication of the need for a free hydroxyl group at the 2' position of the taxol side chain for biological activity. Further, acyl substitutes at the 2' position in 2'-acetyltaxol and 2',7-diacetyltaxol were readily hydrolyzed under in Vivo conditions, and both acetylated compounds showed activity in a cell culture bioassay. The lability of the acyl substitutes at the 2' position suggested that 2'-acetyltaxols could serve as pro-drug forms of taxol (generally, a prodrug is a compound which exhibits pharmacologic activity after biotransformation).
Two taxols with increased water solubility were prepared, 2'-(.beta.-alanyl)-taxol (1) and 2'-succinyltaxol (2): ##STR1## The 2'-(.beta.-alanyl)taxol was found to be active in vivo and in vitro, but was unstable. The 2'-succinyltaxol, prepared by the treatment of taxol with succinic anhydride, had a diminished P-388 in vivo activity as compared with taxol. Thus, research efforts were concentrated on further alterations to taxol and to the known derivatives of taxol (e.g., further derivatives of 2'-succinyl taxol).
Deutsch et al., in "Synthesis of Congeners And Prodrugs. 3. Water-Soluble Prodrugs Of Taxol With Potent Antitumor Activity," J. Med. Chem., 32, 788-792 (1989), reported that salts of 2'-succinyltaxol and 2'-glutaryltaxol had improved antitumor activities when compared to the free acids. Since these researchers believed that salts prepared with different counter ions often have substantially different properties, a variety of 2' substituted taxol salts were synthesized and tested. Triethanolamine and N-methylglucamine salts of the 2' substituted taxol derivatives showed greatly improved aqueous solubility and had more activity than sodium salts. Further, a series of 2'-glutaryltaxol salts were found to have higher activity than their 2'-succinyltaxol analogs. In particular, the taxol salt resulting from the coupling of 2'-glutaryltaxol with 3-(dimethylamino)-1-propylamine using N,N'-carbonyldiimidazole (CDI) demonstrated good solubility and bioactivity.
Mathew et al., in "Synthesis And Evaluation Of Some Water-Soluble Prodrugs And Derivatives Of Taxol With Antitumor Activity," J. Med. Chem., 35, 145-151 (1992), reported the synthesis and evaluation of some 2'-and 7-amino derivatives of taxol. The methane sulfonic acid salts of both 2'- and 7-amino acid esters of taxol showed improved solubility ranging from 2 to greater than 10 mg/mL. The derivatives 2'-(N,N-dimethylglycyl) taxol and 2'-[3-(N,N-dimethylamino)propionyl]taxol inhibited proliferation of B16 melanoma cells to an extent similar to that of taxol, while other derivatives were about 50% as cytotoxic.
Zhao and Kingston, in "Modified Taxols 6. Preparation of Water-Soluble Prodrugs of Taxol," J. Nat. Prod., 54, 1606-1611 (1991), showed that 2'-[(3-sulfo-1-oxopropyl)oxy]taxol sodium salt, 2'-([4-((2-sulfoethyl)amino)-1,4-dioxobutyl]oxy)taxol sodium salt, and 2'-([4-((3-sulfopropyl)amino-1,4-dioxobutyl]oxy)taxol sodium salt have improved water-solubility as compared with taxol and are active as antineoplastic agents in mice. In addition to the above compounds, Kingston and Zhao in U.S. Pat. No. 5,059,699 also describe the synthesis of 2'-.gamma.-aminobutyryltaxol formate and ethylene glycol derivatives of 2'-succinyltaxol.
Taxol prodrugs prepared to date include taxol derivatives having an added aliphatic carboxylic acid moiety. These compounds, however, are readily hydrolysed back to taxol by mild base, and are thus relatively unstable compounds. U.S. Pat. No. 4,942,184 describes the synthesis of several 2' aliphatic carboxylic acid derivatives of taxol. U.S. Pat. No. 4,942,184 also discloses the synthesis of 2'-orthocarboxybenzoyl taxol, although the 2' glutarate series of taxol derivatives are reported to be preferred. It is believed that short aliphatic 2' substituents (e.g., succinyl and glutaryl) were preferred for coupling taxol to solubilizing functionalities, since longer aliphatic or cyclic substituents would lower water solubility. Thus efforts have been concentrated on substituting functional groups on taxol having water solubilizing groups coupled to taxol through short aliphatic chains (e.g. C.sub.3-5).
Taxol, taxol congeners, and prodrugs of both are difficult to synthesize due to the large size and complexity of these compounds, the presence of multiple reactive sites, and the presence of many stereospecific sites. Thus, compared to the importance of these compounds, relatively few taxol congeners and prodrugs have been prepared. Nevertheless, for ease of administration and to minimize allergic side effects, it is desirable to synthesize taxol and taxol congener prodrug formulations that are soluble and stable for several hours or more in aqueous solution, but that can be hydrolysed readily in vivo to yield taxol or an antineoplastic taxol congener.
Thus, there remains a need for taxol derivatives which are soluble and stable in water and exhibit antineoplastic activity. There is a corresponding need to develop synthetic routes to preparing prodrugs of taxol and taxol congeners that exhibit these properties.